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Bilateral comparison between NPL and INMETRO using a high temperature fixed point of unknown temperature.

Machin, G; Teixeira, R*; Lu, X*; Lowe, D (2015) Bilateral comparison between NPL and INMETRO using a high temperature fixed point of unknown temperature. Int. J. Thermophysics, 36 (2-3). pp. 327-335.

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There is an on-going requirement to perform scale comparisons at high temperatures (above the silver point) with reduced uncertainties. This is because previous comparisons have been performed with high stability lamps or radiation thermometers - neither of which have been able to achieve the required uncertainty to substantiate claimed radiation thermometry CMCs (Calibration and Measurement Capabilities). Ideally a set of driftless artefacts, which don't have the inherent measurement challenges of lamps or radiation thermometers should be used as this would allow CMCs to be probed at the highest level. In addition these artefacts should be of unknown temperature. Up until now such artefacts did not exist.

High temperature fixed points (HTFPs), consisting of a metal-carbon eutectic, have been the subject of intense study for more than 10 years. The research has come to an advanced state, so much so that the temperatures of some of the HTFPs are sufficiently well known as to render their use as comparison artefacts questionable, as any such exercise would not be blind.

To address this issue doped HTFPs have been developed, as part of doctoral studies of one of the authors, that have their transition temperature altered from that of the eutectic composition. Two Ni-C-Cu cells, denominated no. 7 and no. 8, were constructed by INMETRO with different levels of Cu. These were compared to a reference Ni-C cell (nominal transition temperature 1602 K) and the temperature difference from the pure state determined. These two cells were then transported to NPL and their temperatures measured. At no time were the temperatures of the cells, as measured by INMETRO, communicated to NPL, thus ensuring that the comparison remained blind. The cells were then returned to INMETRO. Although no. 7 cell was broken it was still useable, and both HTFP cells were re-measured to establish their stability as comparison artefacts and during transport. The temperatures measured at INMETRO and NPL were compared and showed very good equivalence of the scale at the fixed point temperatures.

In this paper the results of the comparison of the NPL and INMETRO temperature scales are reported, along with evidence of the stability of the artefacts determined from the repeat measurements. These promising results indicate that it is possible to make HTFPs with altered temperatures which are stable enough to serve as comparison artefacts

Item Type: Article
Keywords: Comparisons, high temperatures, high temperature fixed points, key comparisons, reference artefacts
Subjects: Engineering Measurements
Engineering Measurements > Thermal
Identification number/DOI: 10.1007/s10765-014-1749-3
Last Modified: 02 Feb 2018 13:13
URI: http://eprintspublications.npl.co.uk/id/eprint/6573

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